Exemplary of the preparation of the Group IV, V and VI metal nitrides is the preparation of niobium nitride. The preparation of niobium nitride from ferroniobium (FeNb) has, in the past, been done in a variety of circuitous ways. The high cost of recovering niobium nitride by the use of several of the known processes has deterred their commercial adoption.
Exemplary of the known methods include the production of niobium oxide (Nb.sub.2 O.sub.5) from FeNb in a multiple step process. The niobium oxide can then be metallothermically or carbothermically reduced to yield metal suitable for further purification by melting. The metal can then be nitrided directly or the Nb.sub.2 O.sub.5 can be nitrided with ammonia.
The typical extraction processes include chlorinating FeNb directly to produce ferric chloride (FeCl.sub.3) and niobium pentachloride (NbCl.sub.5) ) by passing chlorine through a bed of particulate FeNb held at a temperature of from about 500.degree. C. to 1000.degree. C.
The chlorination reaction can be characterized as follows: EQU FeNb+4Cl.sub.2 .fwdarw.FeCl.sub.3 +NbCl.sub.5 +heat
This reaction is exothermic and once started produces considerable heat and must, therefore, be carefully controlled. The FeCl.sub.3 and NbCl.sub.5 produced must be subsequently separated and this is accomplished by passing the mixed chlorides in the vapor state through a heated bed of sodium chloride (NaCl) where the FeCl.sub.3 forms a eutectic composition with the NaCl and is thereby removed from the vapor process stream. The NbCl.sub.5 can then be subsequently condensed by cooling.
This chlorination step utilizes toxic chlorine gas at elevated temperatures and pressures potentially raising corrosion and, therefore, safety problems. Special equipment is necessary for handling the highly pressurized, corrosive liquid chlorine and it must be carefully vaporized, metered and fed into the reactor. Likewise, the most suitable material for reactor construction is graphite. Graphite is a brittle material which can fracture and fail abruptly after a short time in use in this environment. Further, the vaporized chlorine is normally used in excess to ensure complete reaction with the FeNb and the excess must subsequently be neutralized creating an expensive, undesirable by-product.
The separated condensed NbCl.sub.5 can then be hydrolyzed by its addition to water and then the bath is neutralized and the insoluble product can be dried and then calcined, or calcined directly in a heated kiln in the presence of oxygen to produce Nb.sub.2 O.sub.5. The hydrolysis and neutralization steps can produce undesirable by-products and the drying and calcining steps are both energy intensive and expensive.
The Nb.sub.2 O.sub.5 obtained as described can then be metallothermically reduced with aluminum powder in a batch reaction to form Nb metal according to the following equation: EQU 3Nb.sub.2 O.sub.5 +10Al.fwdarw.6Nb+5Al.sub.2 O.sub.3 +heat
This reaction is very exothermic attaining temperatures in excess of the melting point of the products which are then separated by gravity while in the molten state. While expensive, metallothermic reduction is effective with good yields.
The other methods for Nb extraction from FeNb involve caustic or carbonate fusions, which when leached or washed, give niobium oxide which is fairly pure and may be purified further by chlorination as previously described or other means presently known to the art. Ultimately, the oxide must be metallothermically reduced as previously described, or carbothermically reduced to Nb metal.
The process of reducing Nb.sub.2 O.sub.5 carbothermically is difficult to do on a production basis since doing so requires a large thermal input, vacuum vessels, and a careful balance of carbon to oxygen so that the resulting metal is not contaminated with either carbon or oxygen. If the carbon to oxygen ratio is maintained at nearly stoichiometric amounts, then the reaction proceeds rapidly until only a few percent of either remains unreacted. The reaction then proceeds slowly and it is quite difficult for it to reach completion. For this reason, carbothermic reduction is not currently used commercially.
Another method for extracting Nb from FeNb could theoretically be the direct electron beam melting and purification of FeNb by preferential vaporization of the Fe. This becomes very expensive in practice even though the melting point of FeNb is relatively low, a great amount of electrical power is needed to superheat and vaporize the relatively large amount of iron present. Though possible, it is not economically feasible.
Numerous synthesis techniques for niobium nitride preparation are given in the monograph "Hartstoffe", by Kieffer and Benesovsky (Springer-Verlag, Vienna, 1963), p. 317. Among the techniques reviewed therein, the following are noteworthy here.
First, partial hydrogen reduction of Nb.sub.2 O.sub.5 to Nb.sub.2 O.sub.3 can be employed followed by nitriding of the Nb.sub.2 O.sub.3 in the presence of carbon. The potential of such a process is limited by the availability of high quality Nb.sub.2 O.sub.5, and the previously described process difficulties in preparing Nb.sub.2 O.sub.5 commercially suitable for such use.
Next, direct combination of pure niobium metal with nitrogen or ammonia can be employed. Since pure niobium metal is the penultimate product of niobium extractive metallurgy, such a route necessarily represents the most expensive possible for the subsequent production of the nitride.
Further, nitridation of Nb.sub.2 O.sub.5 in the presence of carbon can be employed, as reported by Krishnamurthy, et al R&HM, March 1984, pages 41-45. This process requires difficult pyrovacuum treatment to remove both carbon and oxygen. Furthermore, since that process is intended to ultimately produce niobium metal, the nitride it produces has large amounts of oxygen and carbon.
Still further, it is possible to produce niobium nitride by the direct reaction of Nb.sub.2 O.sub.5 with ammonia. U.S. Bureau of Mines Reports of Investigation 8075 (1975) and 8103 (1976) describes this work, which starts with costly Nb.sub.2 O.sub.5 and ammonia and can also be used to produce niobium metal.